An innovative combination of various recently described molecular methods was set up to efficiently identify regions flanking a marker DNA in insertional mutants of Chlamydomonas. The technique is named restriction enzyme site-directed amplification PCR (RESDA-PCR) and is based on the random distribution of frequent restriction sites in a genome and on a special design of primers. The primer design is based on the presence of a restriction site included in a low degenerated sequence at the 3' end and of a specific adapter sequence at the 5' end, with the two ends being linked by a polyinosine bridge. Specific primers of the marker DNA combined with the degenerated primers allow amplification of DNA fragments adjacent to the marker insertion by using two rounds of either short or long cycling procedures. Amplified fragments from 0.3 to 2 kb or more are routinely obtained at sufficient purity and quantity for direct sequencing. This method is fast, is reliable (87% success rate), and can be easily extrapolated to any organism and marker DNA by designing the appropriate primers. A procedure involving the PCR over enzyme digest fragments is also proposed for when, exceptionally, positive results are not obtained.